Conduct serial analyses for an unusual enzyme, terminal deoxynucleotidyl transferase, on the peripheral blood lymphocytes and bone marrow of approximately 200 leukemia patients. Approximately 200 non-leukemic patient samples will be analyzed for normal control data. Analyses will be carried out at the rate of approximately 30-40 determinations per week (1500 to 2000 determinations per year). The object of the analysis is to evaluate terminal deoxynucleotidyl transferase as a marker for disease state, particularly in acute lymphoblastic leukemia of children.